1. Technical Field
There is a well-known requirement for a method capable of imaging the activity of a nervous system such as a human brain which is sufficiently fast to capture neural activity with sub-second resolution. For example, Susan Greenfield, a professor of pharmacology at Oxford University, England, giving the Andrew Olle Memorial Trust lecture in Sydney, Australia in May 2000 stated that “I think that there is the plausible prospect in this century of being able to devise very good imaging techniques that enable us to catch that which we can't catch at the moment because the imaging techniques are too slow”. The present invention is concerned with delivering a method which is capable of imaging the human brain to sub-second resolution.
2. Related Art
As described by Pomfrett C. J. D. and Healy T. E. J. (1995), “Awareness and the Depth of Anaesthesia”, Healy T. E. J and Cohen P. J. (eds) “A Practice of Anaesthesia”, Edward Arnold, pages 864-878, it is well known that activity within the human brain can be monitored using standard EEG approaches. For example auditory, visual and somatosensory evoked responses can be generated which show that there is a latency after the occurrence of a stimulating event such as an audible sound or a light flash before sensory pathways in the brain respond. Typically there is a delay of at least several tens of milliseconds after a stimulating event before the evoked response can be detected by EEG equipment. Furthermore, although EEG equipment is capable of picking up signals showing that some response has been generated, the location of the source of such signals within the brain cannot be accurately determined from the detected signals.
It is known to use MRI and PET techniques to produce images of cerebral activity, but such techniques respond to haemodynamic and/or metabolic recovery processes which occur over time periods of typically many seconds or minutes and cannot therefore be used to image short term neural activity.
Electrical impedance tomography (EIT) has also been proposed as a method of imaging neurological functions within a body. U.S. Pat. No. 5,919,142 describes various EIT systems which have been proposed for measuring changes in impedance taking place within the brain and using those measurements to image the progress of information along circuits within the brain. It is stated that the brain may be stimulated by for example a visual signal and EIT images subsequently reconstructed for each millisecond or so of the recording “window”, thus enabling the resultant action potential processes to be tracked along their pathways in the subject's brain. Although such a theoretical reconstruction to a resolution of milliseconds is discussed, it is conceded in U.S. Pat. No. 5,919,142 that there is no established technique to permit accurate imaging of neuronal depolarisation with millisecond or sub-millisecond time resolution. It is stated that impedance changes associated with action potentials are generally very small and very rapid and the impedance of the tissue as a whole which is interposed between locations at which impedance measuring electrodes must be positioned may not change in proportion to changes in local action potentials.
Individual impedance measurements (or voltage measurements during current injection) take a finite length of time, typically measured in milliseconds, and in order to build up a sufficient number of impedance measurements to enable the generation of a single image of local impedance distributions within the brain a number of individual impedance measurements must be taken. Typically therefore it takes a few hundred milliseconds to collect sufficient impedance measurements to produce a single image of the brain, although it is feasible to measure voltages in parallel during current injection, thus reducing the measurement period to a few tens of milli-seconds.
A further problem encountered with EIT systems when used for brain imaging is that changes of impedance resulting from neural activity within the brain are thought to be relatively small, for example between 0.1 and 1% of baseline impedance. If true, this makes it very difficult to distinguish impedance fluctuations resulting from changes in neural activity from background noise. The approach suggested in U.S. Pat. No. 5,919,142 seeks to improve sensitivity to changes in impedance resulting from neural activity by taking a first set of impedance measurements whilst a first electrical input signal is being applied to the brain for a period of for example 100 milliseconds or more, taking a second set of impedance measurements when a second electrical input signal that is the reverse of the first is applied to the brain, calculating the difference between the two sets of measurements, and generating an image on the basis of the calculated difference. The application of the first and second input signals can be synchronised with the application of separate stimulus signals to the body. The problem with this approach is that there is a 100 millisecond delay between the generation of the two sets of signals which are compared so as to generate the data from which an image is subsequently generated. It is quite clear therefore that such a system cannot be sensitive to changes in impedance resulting from cerebral activity occurring over periods of only a few milliseconds.
U.S. Pat. No. 5,919,142 dates from a priority date of Jun. 22, 1995. Since that date the same research group has continued with research into the use of electric impedance tomography for studying human brain activity. This is indicated by the paper “3-Dimensional Electrical Impedance Tomography of Human Brain Activity”, Tidswell T., Gibson A., Bayford R. H. and Holder D. S., Neuro Image 13, 283-294 (2001). That paper describes the use of EIT to detect local changes in cerebral blood flow and blood volume. The data measurement for each impedance image was recorded over a period of 25 seconds. Before recording measurements, the neural stimulation process was initiated and remained active for several minutes. It is stated that reproducible impedance changes of about 0.5% lasted from 6 seconds after the onset of a stimulus to 41 seconds after stimulus cessation. The described system was however looking at the side-effects of brain activity, that is changes in blood flow and blood volume, rather than the neurological activity of which such changes are merely side effects. Furthermore, although some interesting results were generated, the paper itself concedes that problems of low resolution and reconstruction error remain which must be overcome if EIT is to be used as a fast neuro imaging tool with clear clinical applications. It is stated that a faster EIT system is being tested which will allow more measurements to be made per image but however many measurements are made, it still cannot be expected that the above technique will be sensitive to neuronal or synaptic phenomena occurring over a period of for example only one or a few milliseconds.
It is an object of the present invention to obviate or mitigate at least one of the problems outlined above.